An important cause of the limited quality of signal processing is the background noise that is always present in the output current of the detector. This background noise comprises at least two components. The first component is known as the “dark current”, in particular, the fluctuating current of thermal origin emitted by the detector, even when it is not receiving photons; the value of this current depends on the temperature of the detector and on its bias state. The second component is the “transient decay current”, in particular, the fluctuating current that is manifested for a certain time after the reception of a photon by the detector; in detectors using semiconductor materials, the transient decay current is primarily due to crystal defects in these materials.
Consider the consequences of this background noise for the accuracy of measurements effected by prior art devices operating by integration.
In these devices, the output current of the detector is integrated directly, and a quantity deemed to represent the effect of the background noise is then subtracted from the charge calculated in this way to obtain a value representative of the incident energy proper. This conventional approach is probably motivated by the need to take account of the totality of the incident energy, including low values thereof, whence the absence of any current processing device on the input side of the integrator. However, as explained above, it is not possible to assign an accurate value to the background noise, in particular because of thermal drift, thermal fluctuations and the transient decay phenomenon. Because of this, prior art incident energy measurements are subject to errors, the magnitude of which is difficult to estimate.